Fixes and improvements to gnus-search

Fixes and improvements to gnus-search

* lisp/gnus/gnus-search.el (gnus-search-default-engines): Change type
from a list of two-element lists, to alist. This matches nnir's old
option type, and should make transition easier.
(nnir-imap-default-search-key): Note that variable is obsolete.
(gnus-search-transform-expression): Interpret the "attachment" key as
"body" in imap searches. Allow specifying larger/smaller message size
values in KB or MB units.
(gnus-search-server-to-engine): Fix error in this function, and
clarify somewhat.